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USGS Frequently Asked Questions

What is obsidian?

Answer: Obsidian is a very shiny natural volcanic glass. When obsidian breaks it fractures with a distinct conchoidal fracture. Obsidian is produced when lava cools very quickly. The lava cools so quickly that no crystals can form. When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water. Obsidian is produced in nature in a similar way.

Composition: Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color. Compared with window glass, obsidian is rich in iron and magnesium; tiny (<.005 mm) crystals of iron oxide within the glass cause its dark color. Obsidian is often formed in rhyolite lava flows where the lava cools so fast that crystals do not have time to grow. Glass, unlike crystals, has no regular structure and therefore fractures in smooth conchoidal (curved) shapes. The intersections of these fractures can form edges sharper than the finest steel blades.

Colors: Obsidian is usually black or a very dark green, but it can also be found in an almost clear form. The colors in obsidian result from the oxidation state of the chemical elements within the tiny minerals that are finely dispersed in the glass. Black color results chiefly from magnetite, Fe304. If the obsidian is highly oxidized, then the glass may contain hematite, which provides a reddish hue. Variations in the oxidation state of the iron (Fe) varieties imparts a slight greenish hue. Some obsidian is banded, a consequence of oxidation on a flow surface being folded into the lava as it continues to move.

Arrowheads and Cutting Tools: Ancient people throughout the world have used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds. Today obsidian is used as a scalpel by doctors in very sensitive eye operations.